1. Field of the Invention
The present invention relates to sensors for detecting a substance in liquid using a surface acoustic wave element (SAW element) and to apparatuses for detecting a substance in liquid including the sensor. More specifically, the present invention relates to a sensor for detecting a substance in liquid that has a reaction film covering the sensing portion of the SAW element and capable of binding a target substance, and to an apparatus for detecting the substance in liquid including the sensor.
2. Description of the Related Art
Various types of sensors have been proposed for detecting substances in liquid.
For example, Japanese Unexamined Patent Application Publication No. 63-250560 (Patent Document 1) discloses a sensor using surface acoustic waves for a substance in liquid. FIG. 16 is a schematic front sectional view of an in-liquid substance detection sensor disclosed in Patent Document 1.
In FIG. 16, the in-liquid substance detection sensor 102 is immersed in liquid 101 including a target substance. The in-liquid substance detection sensor 102 includes a surface acoustic wave element. More specifically, the in-liquid substance detection sensor 102 includes a rectangular piezoelectric substrate 103, and an input IDT electrode 104 and an output IDT electrode 105 that are disposed on a surface of the piezoelectric substrate 103 with a predetermined distance therebetween. A film 106 for adsorbing a substance to be measured is disposed between the input IDT electrode 104 and the output IDT electrode 105. By applying an alternating voltage to the input IDT electrode 104, surface acoustic waves are excited on the piezoelectric substrate 103. The surface acoustic waves propagate toward the output IDT electrode 105. The output IDT electrode 105 receives a signal according the propagated surface waves and the signal is extracted through the output IDT electrode 105. If the target substance is present, the film 106 adsorbs the target substance, and accordingly, the load of the film 106 on the surface of the piezoelectric substrate 103 varies. Consequently, the propagating surface acoustic waves are changed so that the output extracted from the output IDT electrode 105 is changed due to the presence of the target substance. Thus, the presence or absence and/or the concentration of the target substance can be detected.
However, for the measurement using the in-liquid substance detection sensor 102, the in-liquid substance detection sensor 102 must be immersed in a liquid 101. Accordingly, if the amount of liquid 101 to be measured is small, it is difficult to detect the target substance in the liquid.
If a large amount of liquid is prepared, measuring costs may be increased if the liquid is expensive.
In addition, in the in-liquid substance detection sensor 102, the liquid 101 comes into contact with regions other than the region where surface acoustic waves propagate, that is, the region including electrode pads and bonding wires connected to the IDT electrodes 104 and 105. Therefore, the electrical characteristics can be degraded to undesirably reduce the detection accuracy.
Japanese Unexamined Patent Application Publication No. 5-45339 (Patent Document 2) discloses a method for measuring a detection target substance in a liquid without immersing an in-liquid substance detection sensor including a surface acoustic wave filter in the liquid containing the target substance.
In Patent Document 2, an IDT electrode is provided on a first main surface of the piezoelectric substrate, and a measurement cell for placing the liquid containing the target substance is provided in a second main surface opposite the first main surface. Measurement is performed by placing a liquid in the measurement cell in the second main surface. It is therefore not necessary to immerse the entire in-liquid substance detection sensor in the liquid. In addition, since the liquid does not come into contact with the IDT electrode, the electrical characteristics do not substantially change.
The in-liquid substance detection sensor disclosed in Patent Document 2 does not require a large amount of liquid, or allow liquid to come into contact with the IDT electrode.
In the in-liquid substance detection sensor of Patent Document 2, while the liquid containing the target substance is present in the second main surface of the piezoelectric substrate, the surface acoustic waves propagate very close to the first main surface of the piezoelectric substrate, on which the IDT electrode is provided. The propagation of surface acoustic waves does not accurately respond to whether the liquid is present or absent even if the liquid is present in the second main surface. Accordingly, it is difficult to increase the detection accuracy of the in-liquid substance detection sensor of Patent Document 2.
In addition, the in-liquid substance detection sensor disclosed in Patent Document 2 has another disadvantage in that the leakage component of SH waves, which propagates not only along the surface of the piezoelectric substrate, but also at a certain depth from the surface while dispersing the energy, causes noise which reduces the measurement accuracy.